1. Field of the Invention
The present invention relates to a thin film transistor (hereinafter referred to as TFT) formed on a substrate and to an electronic device using the TFT. In particular, the present invention relates to an insulating film that is provided between a substrate and a semiconductor layer serving as an active layer of a TFT. Such an insulating layer, which is referred to as a blocking layer or a base film, is employed for the purpose of preventing the active layer from being contaminated by an impurity such as an alkali metallic element that is in the substrate. Together with preventing the reduction in the reliability and the deterioration of the TFT that are caused by the contamination of the active layer, the present invention relates to a structure of the insulating film that is suitable for obtaining a TFT having good characteristics and small fluctuations within the substrate.
Typically, a liquid crystal display device may be cited as the electronic device of the present invention. It is to be noted that a semiconductor device as used herein throughout the specification refers to any device which functions by utilizing semiconductor characteristics, and the above-mentioned TFT, the electronic device and electronic equipment having the electronic device mounted therein as a display portion are included in semiconductor devices.
2. Description of the Related Art
In recent years, an active matrix type display device that utilizes a TFT, which has an active layer formed of a crystalline semiconductor layer, as switching elements of pixels and driver circuits is attracting much attention as a means of realizing an incredibly fine and high quality image display. A crystalline silicon layer formed of an amorphous silicon layer that is crystallized by a known method such as laser annealing or thermal annealing may be suitably used as a material of the crystalline semiconductor layer. In the TFTs using the crystalline silicon layer, an electric field effect mobility is high and because a high current driving ability can be attained even if fine processing is performed, it is possible to increase an aperture ratio of a pixel portion.
A quartz glass substrate that does not contain alkali oxide therein and a low alkali glass substrate that contains alkali oxide therein can be used as the substrate of such an active matrix type display device. However, it is preferable that an inexpensive low alkali glass substrate is used rather than the quartz glass substrate in order to realize a low price active matrix type display device. However, in the case of using the low alkali glass substrate as the substrate, the highest temperature in the manufacturing process thereof must be at between 600xc2x0 C. and 700xc2x0 C. in terms of the heat resistance of the glass substrate.
Further, it was necessary to at least form a blocking layer that is made of a silicon oxide film or a silicon nitride film on the side of the surface of the glass substrate on which the TFT will be formed such that a small amount of alkali metal such as sodium (Na) that is contained in the substrate will not mix into the active layer of the TFT. Known structures of TFTs formed on the glass substrate are the top gate type and the bottom gate type (or inverted stagger type). The top gate type has a structure in which at least a gate insulating film and a gate electrode are provided on a surface that is the opposite side of the substrate side of the active layer. In this top gate type TFT, among the alkali metallic elements in the glass substrate, the ones that have been ionized are drawn to the side of the active layer depending on the polarity of the gate electrode when a voltage is applied thereto. Therefore, the blocking layer as mentioned above is formed on a surface that is opposite from the surface where the active layer is in contact with the gate insulating film (hereinafter referred to as back channel side throughout the present specification for the sake of convenience). If the quality of this blocking layer is poor, the alkali metallic elements in the glass substrate will easily mix into the active layer, whereby the electrical characteristic of the TFTs will change. Hence, reliability cannot be secured. In addition, if the blocking layer is provided and an amorphous semiconductor layer is formed thereon to thereby form a crystalline semiconductor layer by laser annealing or thermal annealing, then an internal stress of the blocking layer will change, causing the crystalline semiconductor layer to distort. Even if the TFT is completed under such conditions, the electrical characteristics of the TFT such as a threshold voltage (hereinafter abbreviated as Vth) and a subthreshold constant (hereinafter abbreviated as S-value and referred to as such) will vary from a target value.
Therefore, it is disclosed in Japanese Patent Application No. Hei 11-125392 that a blocking layer made of a lamination of a silicon oxynitride film (A) and a silicon oxynitride film (B) is provided on the back channel side of the TFT to thereby prevent contaminations caused by impurities such as the alkali metallic elements from the substrate. In addition, an appropriate range of the composition and the film thickness of the first layer silicon oxynitride film (A) and the second layer silicon oxynitride film (B) such that the internal stress becomes small before and after the crystallization process of the amorphous semiconductor layer, that is, influences to the crystalline semiconductor layer will be small, is disclosed in the above mentioned application.
The oxygen concentration contained in the silicon oxynitride film (A) is set to between 20 atomic % and 30 atomic %, and the nitrogen concentration is set to between 20 atomic % and 30 atomic %, or the composition ratio of nitrogen to oxygen is set to between 0.6 and 1.5. Furthermore, the oxygen concentration contained in the silicon oxynitride film (B) is set to between 55 atomic % and 65 atomic %, and the nitrogen concentration is set to between 1 atomic % and 20 atomic %, or the composition ratio of nitrogen to oxygen is set to between 0.01 and 0.4. The hydrogen concentration of the silicon oxynitride film (A) is set to between 10 atomic % and 20 atomic %, or the composition ratio of hydrogen to oxygen is set to between 0.3 and 1.5, and the hydrogen concentration of the silicon oxynitride film (B) is set to between 0.1 atomic % and 10 atomic %, or the composition ratio of hydrogen to oxygen is set to between 0.001 and 0.15.
Further, the density of the silicon oxynitride film (A) is set to between 8xc3x971022 atoms/cm3 and 2xc3x971023 atoms/cm3, and the density of the silicon oxynitride film (B) is set to between 6xc3x971022 atoms/cm3 and 9xc3x971022 atoms/cm3. The etching rate of a mixed aqueous solution containing 7.13% of ammonium hydrogen fluoride (NH4HF2) and 15.4% of ammonium fluoride (NH4F) of such a silicon oxynitride film (A) at 20xc2x0 C. is between 60 nm/min and 70 nm/min (after heat treatment at 500xc2x0 C. for 1 hour, and heat treatment at 550xc2x0 C. for 4 hours, the etching rate is between 40 nm/min and 50 nm/min). The etching rate of the silicon oxynitride film (B) is between 110 nm/min and 130 nm/min (after heat treatment at 500xc2x0 C. for 1 hour, and heat treatment at 550xc2x0 C. for 4 hours, the etching rate is between 90 nm/min and 100 nm/min). The etching rate defined here is a value obtained from performing etching at 20xc2x0 C. with an aqueous solution containing 7.13% of NH4HF2 and 15.4% of NH4F as the etching solution.
By providing the silicon oxynitride film (A) in contact with the substrate at a thickness of between 10 nm and 150 nm, preferably between 20 nm and 60 nm and providing the silicon oxynitride film (B) thereon at a thickness of between 10 nm and 250 nm, preferably between 20 nm and 100 nm, the contamination of the active layer by impurities such as alkali metallic elements in the substrate can be prevented.
Furthermore, because the blocking layer is formed by laminating the silicon oxynitride film (A) and the silicon oxynitride film (B), it is preferable that, taking into consideration the internal stress of the blocking layer under the laminated state, the changing amount of the internal stress thereof before and after the crystallization process is 1xc3x97102 N/m2 or less.
The manufacturing method of the silicon oxynitride films at this point may he formed by known a film deposition method such as plasma CVD, low pressure CVD or ECR-CVD. Particularly, it is preferable that plasma CVD is used to form the silicon oxynitride films. Further, SiH4, NH3, and N2O are used as the raw gas. The composition ratio is realized by controlling the supplying amount of the raw gas, or adjusting parameters such as reaction pressure, discharge power, discharge frequency, and substrate temperature that are related to the film deposition of the silicon oxynitride films. NH is a supplement for the nitriding of the silicon oxynitride film. The amount of nitrogen contained in the silicon oxynitride film can be effectively controlled by appropriately adjusting the supplying amount of NH3. Therefore, compared with the silicon oxynitride film (B), the silicon oxynitride film (A) having a high concentration of nitrogen is formed from SiH4, NH3, and N2O whereas the silicon oxynitride film (B) is formed from SiH4 and N2O.
By forming the blocking layer based on the composition and the film thickness of the silicon oxynitride film (A) and the silicon oxynitride film (B) as mentioned above, contamination by the alkali metallic elements from the substrate can be prevented. Thus, the internal stress of the blocking layer can be made small before and after the crystallization process of the amorphous semiconductor layer and influences to the crystalline semiconductor layer can be lessened. Consequently, the electrical characteristics of the TFTs such as the Vth and the S-value become the target value, whereby highly reliable TFTs can be manufactured.
The above-mentioned Vth, which is a typical parameter of the electrical characteristic of TFTs, in the characteristic of the xc2xd power of a drain current (hereinafter referred to as Id) versus a gate voltage (hereinafter referred to as Vg), can be obtained as a voltage value that intersects a Vg axis by extrapolating a linear region. Also, the relationship between the drain current and the gate voltage in the vicinity of the Vth or less is called the subthreshold characteristic, which is a very important characteristic for determining the performance of the TFT as a switching-element. The S-value is used as a constant for expressing the goodness of the subthreshold characteristic. Further, the S-value is defined as the gate voltage that is required for the drain current to make a 1 line change when the subthreshold characteristic is plotted to a single logarithm graph.
For the value of the Vth to operate a circuit, an N channel TFT may be set to between 0.5 V and 2.5 V, and a P channel TFT may be set to between xe2x88x922.5 V and xe2x88x920.5 V. It is to be noted that a method of doping an impurity element that imparts a p-type conductivity into a channel forming region of the active layer at a concentration of about 1xc3x971016 atoms/cm3 to 5xc3x971017 atoms/cm3 is taken in order to control the Vth. Such measure is called a channel doping and is a crucial process for attaining a switching characteristic of the TFTs according to the design.
In addition, the smaller the S-value is, the width of the voltage that is necessary for switching between the ON state and the OFF state of the TFTs can be small, thereby making it possible for the TFTs to operate rapidly and at a low consumption power.
Fluctuations of the values of the electrical characteristics of the TFTs such as the Vth and the S-value may be seen in the TFTs inside the surface of the substrate. As the fluctuations of these characteristic values become large, then a large margin of the gate voltage must be taken. As a result, the voltage that is necessary for operation becomes high, thereby increasing the consumption power. Further, the values of the Vth and the S-value remarkably influence the reproductivity of the gradation display of a liquid crystal display device, particularly the reproductivity of an intermediate color. Therefore, if these values fluctuate, then the original display contents cannot be genuinely reproduced when displayed, and an inconsistent display may be recognized.
The present invention has been made in view of the above problem, and therefore has an object to provide a TFT that has a small fluctuation in a typified characteristic of the TFT and a method of manufacturing the same to thereby provide an active matrix type liquid crystal display device employing such a TFT.
For the purpose of resolving the above problem., the present inventor conducted many experiments regarding the relationship between the fluctuations of the electrical characteristics of the TFT typified by the Vth and the S-value and the processes necessary for manufacturing the TFT. The conclusion after many trials and errors was that in the above-mentioned structure, which has the provision of a blocking layer formed by laminating a silicon oxynitride film (A) and a silicon oxynitride film (B) on the side of a back channel, the present inventor discovered that there was a close relationship between the fluctuations of the electrical characteristics of the TFT and the structure of the blocking layer.
Furthermore, in the above-mentioned structure having the provision of the blocking layer formed by laminating the silicon oxynitride film (A) and the silicon oxynitride (B) on the side of the back channel, the present inventor discovered, upon proceeding with the experiments, that the fluctuations of the electrical characteristics of the TFT are influenced more by the film thickness, the quality of the film, and the film uniformity of the silicon oxynitride film (B) than by those of the silicon oxynitride film (A).
In regards to evaluating the fluctuation of the electrical characteristics of the TFT, attention is directed to the characteristic for normal operating a circuit that is formed of a TFT at a desirable driving voltage, that is, the values of Vth. S-value, and electric field effect mobility. Therefore, attention is particularly directed to the values of Vth and S-value here. Shown in FIGS. 1 and 2 is the influence of the film thickness of the silicon oxynitride film (B) imparted to the fluctuation of the Vth and the S-value. FIG. 1 shows the fluctuation of the Vth and FIG. 2 shows the fluctuation of the S-value. The shape of each of the plots indicates the film thickness of the silicon oxynitride film (B), where the plot denoted by ◯ is 30 nm, the plot denoted by xcex94 is 50 nm, and the plot denoted by xe2x96xa1 is 100 nm. As shown in the figures, as the slope of the straight line formed by each of the plots becomes larger, the fluctuations of the respective values become smaller. For example, it is apparent from FIG. 1 that the fluctuation of the Vth becomes larger as the film thickness of the silicon oxynitride film (B) sequentially becomes 50 nm, 30 nm, and 100 nm. 99% of the Vth value of a TFT is in the range of xe2x88x921.78 V to xe2x88x921.59 V when the film thickness of the silicon oxynitride film (B) is 50 nm, fluctuation is in the range of xe2x88x921.77 V to xe2x88x921.46 V when the film thickness thereof is 30 nm, and fluctuation is in the range of xe2x88x922.51 V and xe2x88x921.82 V when the film thickness thereof is 100 nm. In addition, when the film thickness of the silicon oxynitride film (B) is 50 nm, it is also apparent that 99% of the Vth value of the TFT is held within a width of 0.19 V, that is, a value obtained by subtracting xe2x88x921.78 V from xe2x88x921.59 V. The Vth value thus obtained is referred as the 99% fluctuation width of Vth. It is to be noted that the film thickness of the silicon oxynitride film (A) is all 50 nm. Thus, the fluctuation was evaluated by measuring 100 P channel TFTs of the same size dispersed at approximately equal intervals in a 10 cmxc3x9710 cm substrate.
Shown in FIG. 3 is the relationship between the 99% fluctuation width of Vth and the film thickness of the silicon oxynitride film (B). Compared with the case where the film thickness of the silicon oxynitride film (B) is 100 nm, the fluctuation of the electrical characteristics of the TFT has become remarkably smaller when the film thickness thereof thinner, that is, 50 nm. Further, when the film thickness of the silicon oxynitride film (B) is 30 nm, the fluctuation of the electrical characteristics is smaller when compared with the 100 nm thick silicon oxynitride film (B) but somewhat larger when the film thickness thereof is 50 nm. Therefore, it can be discerned that the fluctuation of the electrical characteristics of the TFT has become effectively smaller by making the film thickness of the silicon oxynitride film (B) thinner from 100 nm to 50 nm. However, it can also be discerned that when the film thickness thereof is further made thinner from 50 nm to 30 nm, the uniformity of the film quality is reduced, resulting in making the fluctuation larger.
Therefore, in order to effectively make the fluctuation of the electrical characteristic of the TFT smaller, it is appropriate to form on a 10 cmxc3x9710 cm substrate the silicon oxynitride film (B) whose film thickness is between 30 nm and 70 nm, preferably 50 nm, that is, a film thickness where the 99% fluctuation width of Vth is within approximately 0.3 V.
On the other hand, as the film thickness of the silicon oxynitride film (A) is made thicker, the effect of preventing contamination from the substrate becomes higher. However, if the film thickness of the silicon oxynitride film (A) is thicker than 100 nm, there are cases where an amorphous silicon film that will be formed thereon peels off. In addition, it has been confirmed that the effect of the, silicon oxynitride film (A) in preventing contamination by alkali metallic elements is sufficient, even when the film thickness thereof is at 50 nm. Therefore, the film thickness of the silicon oxynitride film (A) is preferable between 50 nm and 100 nm. Thus, there is an optimal range for the film thickness and film quality of the blocking film that uses the silicon oxynitride film provided on the side of the back channel, and by adopting an appropriate combination thereof, not only can the characteristic of the TFT be stabilized but the fluctuation in the Vth and the S-value can be reduced. As a result, the driving voltage of the liquid crystal display device can be reduced, whereby the consumption power is also reduced, and the reproductivity of the gradation display of the liquid crystal display device can be further enhanced.